Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by amoult, in which the hardexoskeleton is shed to allow the animal to grow. Thelarvae ofcrustaceans often bear little resemblance to the adult, and there are still cases where it is not known what larvae will grow into what adults. This is especially true of crustaceans which live as benthic adults (on the sea bed), more-so than where the larvae areplanktonic, and thereby easily caught.
Many crustacean larvae were not immediately recognised as larvae when they were discovered, and were described as new genera and species. The names of these genera have become generalised to cover specific larval stages across wide groups of crustaceans, such aszoea andnauplius. Other terms described forms which are only found in particular groups, such as theglaucothoe ofhermit crabs, or thephyllosoma ofslipper lobsters andspiny lobsters.
At its most complete, a crustacean's life cycle begins with anegg, which is usuallyfertilised, but may instead be produced byparthenogenesis. This egg hatches into a pre-larva or pre-zoea. Through a series of moults, the young animal then passes through various zoea stages, followed by a megalopa or post-larva. This is followed bymetamorphosis into an immature form, which broadly resembles the adult, and after further moults, the adult form is finally reached. Some crustaceans continue to moult as adults, while for others, the development ofgonads signals the final moult.
Any organs which are absent from the adults do not generally appear in the larvae, although there are a few exceptions, such as thevestige of the fourthpereiopod in the larvae ofLucifer, and somepleopods in certainAnomura andcrabs.[1] In a more extreme example, theSacculina and otherRhizocephala have a distinctive nauplius larva with its complex body structure, but the adult form lacks many organs due to extreme adaptation to its parasitic life style.
Antonie van Leeuwenhoek was the first person to observe the difference between larval crustaceans and the adults when he watched the eggs ofCyclops hatching in 1699.[1] Despite this, and other observations over the following decades, there was controversy among scientists about whether or notmetamorphosis occurred in crustaceans, with conflicting observations presented, based on different species, some of which went through a metamorphosis, and some of which did not. In 1828John Vaughan Thompson published a paper "On the Metamorphoses of the Crustacea, and on Zoea, exposing their singular structure and demonstrating they are not, as has been supposed, a peculiar Genus but the Larva of Crustacea!!" However his work was not believed due to crayfish not undergoing metamorphosis.[2] This controversy persisted until the 1840s, and the first descriptions of a complete series of larval forms were not published until the 1870s (Sidney Irving Smith on theAmerican lobster in 1873;Georg Ossian Sars on theEuropean lobster in 1875, andWalter Faxon on the shrimpPalaemonetes vulgaris in 1879).[1]
The genus nameNauplius was published posthumously byOtto Friedrich Müller in 1785 for animals now known to be the larvae ofcopepods. The nauplius stage (plural:nauplii) is characterised by consisting of only three head segments, which are covered by a singlecarapace. The posterior body, when present, is unsegmented. Each head segment has a pair ofappendages; theantennules, antennae, andmandibles. This larval stage has various lifestyles; some are benthic while others are swimmers, some are feeding while others are non-feeders (lecithotrophic). The nauplius is also the stage at which a simple, unpaired eye is present. The eye is known for that reason as the "naupliar eye", and is often absent in later developmental stages, although it is retained into the adult form in some groups, such as theNotostraca.[3][4] Some crustacean groups lack this larval type,isopods being one example.[5]
The genusZoea was initially described byLouis Augustin Guillaume Bosc in 1802 for an animal now known to be the larva of acrab.[1] The zoea stage (plural:zoeas orzoeae), only found in members ofMalacostraca,[5] is characterised by the use of thethoracic appendages for swimming and a large dorsal spine.[5]
The post-larva orMegalopae, also found exclusively in the Malacostraca,[5] is characterised by the use of abdominal appendages (pleopods) for propulsion. The post-larva is usually similar to the adult form, and many names have been erected for this stage in different groups.William Elford Leach erected the genusMegalopa in 1813 for a post-larval crab; acopepod post-larva is called acopepodite; abarnacle post-larva is called acypris; ashrimp post-larva is called aparva; ahermit crab post-larva is called aglaucothoe; aspiny lobster /furry lobsters post-larva is called apuerulus and aslipper lobster post-larva is called anisto.
In theBranchiopoda, the offspring hatch as a nauplius or metanauplius larva.[6]
In theMediterraneanhorseshoe shrimpLightiella magdalenina, the young experience 15 stages following the nauplius, termedmetanaupliar stages, and two juvenile stages, with each of the first six stages adding two trunk segments, and the last four segments being added singly.[7]
The larvae ofremipedes arelecithotrophic, consumingegg yolk rather than using external food sources. This characteristic, which is shared withmalacostracan groups such as theDecapoda andEuphausiacea (krill) has been used to suggest a link between Remipedia and Malacostraca.[8]
Amphipod hatchlings resemble the adults.[9]
Youngisopod crustaceans hatch directly into amanca stage, which is similar in appearance to the adult. The lack of a free-swimming larval form has led to high rates ofendemism in isopods, but has also allowed them to colonise the land, in the form of thewoodlice.
The larvae of many groups ofmantis shrimp are poorly known. In the superfamilyLysiosquilloidea, the larvae hatch asantizoea larvae, with five pairs of thoracic appendages, and develop intoerichthus larvae, where the pleopods appear. In theSquilloidea, apseudozoea larva develops into analima larva, while inGonodactyloidea, apseudozoea develops into anerichthus.[10]
A singlefossil stomatopod larva has been discovered, in theUpper JurassicSolnhofenlithographic limestone.[11]
The life cycle of krill is relatively well understood, although there are minor variations in detail from species to species. After hatching, the larvae go through several stages callednauplius,pseudometanauplius,metanauplius,calyptopsis andfurcilia stages, each of which is sub-divided into several sub-stages. Thepseudometanauplius stage is exclusive to the so-called "sac-spawners". Until themetanauplius stage, the larvae are reliant on theyolk reserves, but from thecalyptopsis stage, they begin to feed onphytoplankton. During thefurcilia stages, segments with pairs of swimmerets are added, beginning at the frontmost segments, with each new pair only becoming functional at the next moult. After the final furcilia stage, the krill resembles the adult.
Apart from the prawns of the suborderDendrobranchiata, all decapod crustaceans brood their eggs on the female's pleopods. This has resulted in development in decapod crustaceans being generally abbreviated.[1] There are at most nine larval stages in decapods, as inkrill, and both decapod nauplii and krill nauplii often lackmouthparts and survive on nutrients supplied in theegg yolk (lecithotrophy). In species with normal development, eggs are roughly 1% of the size of the adult; in species with abbreviated development, and therefore more yolk in the eggs, the eggs may reach 1/9 of the adult's size.[1]
The post-larva ofshrimp is calledparva, after the speciesAcanthephyra parva described byHenri Coutière, but which was later recognised as the larva ofAcanthephyra purpurea.[12]
In the marinelobsters, there are three larval stages, all similar in appearance.
Freshwatercrayfishembryos differ from those of other crustaceans in having 40ectoteloblast cells, rather than around 19.[13] The larvae show abbreviated development, and hatch with a full complement of adult appendages with the exceptions of theuropods and the first pair ofpleopods.[1]
The larvae of theAchelata (slipper lobsters,spiny lobsters andfurry lobsters) are unlike any other crustacean larvae. The larvae are known asphyllosoma, after the genusPhyllosoma erected byWilliam Elford Leach in 1817. They are flattened and transparent, with long legs and eyes on long eyestalks. After passing through 8–10 phyllosoma stages, the larva undergoes "the most profound transformation at a single moult in the Decapoda", when it develops into the so-calledpuerulus stage, which is an immature form resembling the adult animal.[1]
The members of the traditional infraorderThalassinidea can be divided into two groups on the basis of their larvae. According toRobert Gurney,[1] the "homarine group" comprises the familiesAxiidae andCallianassidae, while the "anomuran group" comprises the familiesLaomediidae andUpogebiidae. This split corresponds with the division later confirmed withmolecular phylogenetics.[14]
Among theAnomura, there is considerable variation in the number of larval stages. In theSouth Americanfreshwater genusAegla, the young hatch from the eggs in the adult form.[1]Squat lobsters pass through four, or occasionally five, larval states, which have a longrostrum, and a spine on either side of thecarapace; the first post-larva closely resembles the adult.[1]Porcelain crabs have two or three larval stages, in which the rostrum and the posterior spine on the carapace are "enormously long".[1]Hermit crabs pass through around four larval stages. The post-larva is known as theglaucothoe, after a genus named byHenri Milne-Edwards in 1830.[1] The glaucothoe is 3 millimetres (0.12 in) long inPagurus longicarpus, but glaucothoe larvae up to 20 mm (0.79 in) are known, and were once thought to represent animals which had failed to develop correctly.[1] Like the preceding stages, the glaucothoe is symmetrical, and although the glaucothoe begins as a free-swimming form, it often acquires agastropod shell to live in; thecoconut crab,Birgus latro, always carries a shell when the immature animal comes ashore, but this is discarded later.[1]
Although they are classified ascrabs, the larvae ofDromiacea are similar to those of the Anomura, which led many scientists to place dromiacean crabs in the Anomura, rather than with the other crabs. Apart from the Dromiacea, all crabs share a similar and distinctive larval form. The crab zoea has a slender, curvedabdomen and a forkedtelson, but its most striking features are the long rostral and dorsal spines, sometimes augmented by further, lateral spines.[1] These spines can be many times longer than the body of the larva. Crab prezoea larvae have been foundfossilised in thestomach contents of theEarly Cretaceousbony fishTharrhias.[15]
Copepods have six naupliar stages, followed by a stage called thecopepodid, which has the same number of body segments and appendages in all copepods. The copepodid larva has two pairs of unsegmented swimming appendages, and an unsegmented "hind-body" comprising the thorax and the abdomen.[1] There are typically five copepodid stages, but parasitic copepods may stop after a single copepodid stage. Once the gonads develop, there are no further moults.[1]
Chalimus (plural chalimi) is a stage of development of a copepod parasite of fish, such as thesalmon louse (Lepeophtheirus salmonis).[17][18]
Chalimus Burmeister, 1834 is also a synonym forLepeophtheirus Nordmann, 1832.
The single genus in theFacetotecta,Hansenocaris, is only known from its larvae. They were first described byChristian Andreas Victor Hensen in 1887, and named "y-nauplia" byHans Jacob Hansen, assuming them to be the larvae ofbarnacles.[19] The adults are presumed to beparasites of other animals.[20]
